Heating, ventilation and air conditioning (HVAC) systems can be a significant contributor to the energy requirements of a vehicle. The high energy demand of the compressor in these systems can lead to a reduction of 8% to 12% in the mileage range of an internal combustion engine (ICE) vehicle, due to the impact on fuel economy. In an electric vehicle (EV), the HVAC system can lead to a reduction of 40% in the mileage range. Accordingly, reducing the energy load of HVAC systems is of considerable interest, as this has the potential to provide benefits associated with improved energy efficiency and fuel economy. Increasing the range of electric vehicles is also desirable.
Reference 1 discusses the impact of vehicle air conditioning on fuel economy, tailpipe emissions and electric vehicle range. In order to reduce air conditioning load, this document proposes a number of options, including advanced glazing to reduce the effect of solar energy on heating a vehicle cabin, and using recirculated air in the HVAC system. Operating an HVAC system in recirculation mode, where air from the cabin is passed through the HVAC system and returned to the cabin, can considerably reduce the energy load of the HVAC system. This reduces the amount of air brought in from outside the vehicle for ventilation, which reduces the load on the HVAC system since the recirculated cabin air is typically closer to the desired temperature than air from outside the cabin. Reference 1 highlights that there are disadvantages associated with operating the HVAC system in recirculation mode, discussing in particular that a build-up of volatile organic compounds (VOCs) is observed when HVAC systems are operated in recirculation mode (especially in newer cars). Reference 1 proposes using 70% recirculated air for vehicle heating, and 80% recirculated air for vehicle cooling.
A further advantage of operating the HVAC system of a vehicle in recirculation mode is that this can reduce the concentration of particulate pollutants in the cabin, as discussed in References 2 and 3. As Reference 2 explains, airborne nanoparticles from vehicle emissions pose health risks, particularly when driving in heavy traffic. This document reports that the concentration of nanoparticles is rapidly significantly reduced in two commercially available vehicles when their HVAC systems are operated in recirculation mode while driving in heavy traffic. This effect is also discussed in Reference 3.
However, Reference 3 highlights a significant problem associated with the operation of HVAC systems in recirculation mode. In recirculation mode, carbon dioxide levels inside the cabin rise very rapidly, primarily due to exhalation of carbon dioxide by the vehicle's occupants. Reference 3 is concerned with modelling the carbon dioxide concentration in vehicle cabins. This document discusses two possible approaches for supressing the increase of cabin carbon dioxide concentration observed when recirculation mode is employed. The first is automatic air recirculation, which relies on a fast-responding gas sensor to detect high levels of pollutants on roadways. On detection of high pollution levels, air intake is temporarily shut off and cabin air recirculated. A similar system is described in US2005/0217487, where air recirculation and contaminant removal is triggered in response to contaminants detected by a sensor, e.g. in a building or an agricultural vehicle. This document proposes removing carbon dioxide from the cabin when recirculation has been activated, using an assembly for adsorbing carbon dioxide.
The second approach proposed in Reference 3 is fractional continuous recirculation, where some external air intake is permitted to suppress the increase in carbon dioxide concentrations. Reference 4 reports measurements of particulate concentration and carbon dioxide levels under fractional air recirculation conditions.
US2006/0144061 proposes a climate control system which, in its normal operation mode is operated in recirculation mode. Upon exceeding a pre-determined carbon dioxide threshold value measured in the passenger compartment of the motor vehicle the climate control system is switched to fresh air mode.